As for an inverter that receives DC voltage and outputs three-phase AC voltage, technology for detecting line current of each phase flowing between the inverter and a load, and controlling operation of the load based on the line current is known.
Three-phase current flows between the inverter and the load, and thus, if line current of two phases is detected, line current of the other phase is determined unambiguously. At least two sensors each detecting line current are therefore usually necessary.
Use of a plurality of sensors, however, leads to high cost. To solve this problem, there is technology of providing a sensor to at least one of a pair of DC buses for supplying DC voltage to the inverter to detect current (hereinafter, referred to as “DC link current”) flowing through the sensor as line current of one phase. Specifically, which phase of line current the DC link current corresponds to is determined by taking a switching pattern used by the inverter while the DC link current flows into consideration.
In this technology, in order to determine which phase of line current the DC link current corresponds to, one switching pattern has to be maintained for time required to detect the DC link current.
The DC link current is commutated in current paths in the inverter due to switching of the inverter, and then ringing is generated. There is turn-on time of a switching device for performing switching of the inverter. Time for A/D conversion is further required to handle values of the DC link current as data.
That is to say, time for which one switching pattern has to be maintained to associate the DC link current with line current of one phase has to be at least longer than the total time of time for ringing of the DC link current, time for A/D conversion, and time for turning-on of the switching device. Still another factor, such as dead time of switching, may have to be considered.
There is, therefore, a minimum amount of time required to maintain a switching pattern to measure the DC link current to detect it as line current. This value is hereinafter referred to as a minimum time limit.
A compulsory increase, solely, in time for which a switching pattern is used to obtain line current obviously causes distortion of output voltage of the inverter, and further causes distortion of line current output from the inverter.
The switching pattern of the inverter, especially a voltage source inverter, can be determined using a so-called method with a circular locus (or a circular approximation method). The method with a circular locus (hereinafter “the circular locus method”) is well known from Japanese Patent No. 3289281 and Ohyama and four others, “A Novel Current Control Technique for Voltage Source Inverter in Field Oriented Control”, the Institute of Electrical Engineers of Japan Transactions B, the Institute of Electrical Engineers of Japan, 1985, Vol. 105, No. 11, p. 901-908, Daijyo and three others, “On the Magnetic Noise of an Induction Motor Driven by PWM Inverter”, the Institute of Electrical Engineers of Japan Transactions D, the Institute of Electrical Engineers of Japan, 1988, Vol. 108, No. 3, p. 237-244 and Daijyo and three others, “On PWM Patterns and Harmonic Analysis of a General Purpose Inverter for Induction Motor Drives”, the Institute of Electrical Engineers of Japan Transactions D, the Institute of Electrical Engineers of Japan, 1989, Vol. 109, No. 11, p. 809-816, for example. In the circular locus method, magnetic flux in the load is controlled every predetermined control cycle.
Further, Japanese Patent Application Laid-Open Publication No. 11-4594, Japanese Patent No. 3664040, Japanese Patent No. 4643404 and Japanese Patent Application Laid-Open Publication No. 2011-234428 are also listed as prior art documents to describe problems later.